Injection mixers for adding a fluid, i.e. liquid, gas and/or a suspension of liquid and solids, to a process liquid have been known for decades. As an example of modern injection mixers, chemical mixers of Wetend Technologies Oy may be discussed. Patent documents EP-B1-1064427, EP-B1-1219344, FI-B-111868, FI-B-115148 and FI-B-116473 discuss such mixers and explain how injection mixing is used in the introduction and mixing of a fluid into a process liquid. The injection mixing is performed by not only spraying the fluid itself to the process liquid, but an injection liquid is used for enhancing the penetration of the liquid-fluid mixture deep into the process liquid. Sometimes also another liquid, i.e. mixing liquid is used. The injection liquid is not the fluid to be mixed, but another fluid the amount of which is normally higher than that of the fluid to be mixed. The injection liquid may be the same as the process liquid, or it may be some other, though applicable, process liquid, it may be a filtrate from somewhere from the process, or it may be clear water, for instance. The mixing liquid may be yet another liquid, which is normally used for diluting the fluid or chemical to be mixed prior to the introduction of the fluid into communication with the injection liquid. The mixing liquid may, for instance, be chosen from the same list of liquids as the injection liquid. The injection mixing station, is formed of injection mixers arranged on substantially the same circumference of the process pipe or reactor so that one mixer is sufficient for pipes/reactors of small diameter, slightly larger pipes/reactors use two opposing mixers on the same circumference, pipes/reactors slightly larger than this need three mixers located at 120 degree intervals on the circumference and so on. In other words, usually more than one mixer, i.e. a mixing station is needed to inject the fluid to the process liquid in mill scale applications. Sometimes such a mixing station may include another set of injection mixers arranged on another circumference of the flow pipe, however, axially close to the first group of mixers.
The present practice, i.e. the latest and the most advanced way of introducing injection and mixing liquid (if used) to the injection mixers, shown in FIG. 1, has been to arrange the inlet opening of an injection liquid feed pump in communication with a source of appropriate liquid, and to pump the liquid further to a manifold, so called injection liquid manifold, which divides the liquid to the injection mixers. In view of the successful operation of the mixing station it is important that each mixer injects the same amount of fluid/chemical with the same amount of injection liquid. However, the practice has shown that when using manifolds that is not always the case, i.e. the manifolds do not very easily divide the injection liquid equally to the feed lines leading to the injection mixers. In order to ensure that the manifold shares the injection liquid equally enough to the feed lines, a considerable amount of R&D, design, construction and testing work has to be performed for each particular manifold. Further, it has to be understood that the manifold design is highly dependent on the volume flows, whereby a manifold designed for dividing a certain volume flow evenly is not necessarily able to share a volume flow 20% higher or lower evenly enough anymore. Thus it has been learned that the manifold is, in practice, always a compromise between the expenses related to its design, R&D, construction and testing and the equal enough sharing of the injection liquid.
The feed lines from the manifold to the injection mixers are provided with valves but the valves are on-off-type valves and they cannot be used for controlling the volume flow. The main reason for the use of the above type of valves is that the injection liquid is not normally clean liquid but contains particulate matter. This is self-evident as the process liquid itself may very often be used as the injection liquid. If the process liquid is, for instance a paper making stock, it may contain fibers and fillers that would easily, when throttling the flow in a valve, collect or accumulate in places where the flow is slow and less turbulent. After some time the accumulated fibers and/or fillers, more generally accumulated impurities, may loosen and flow along with the flow into the injection mixer. Such accumulated impurities may impede the flow in the injection mixer, block the mixer or at least enter the process liquid and cause problems in the end product or its production. Thus the manifold or a corresponding means for dividing the injection liquid should be capable of dividing the injection liquid evenly to the feed pipes taking the injection liquid to the injection mixers.
If mixing liquid is used, it is common practice to use the liquid used as the injection liquid as the mixing liquid, too, though other liquids may also be used. However, in case it is the injection liquid that is used as the mixing liquid, the through-flow of the injection liquid manifold is usually taken to another manifold, so called mixing liquid manifold that divides the mixing liquid to the injection mixers. The feed lines from the mixing liquid manifold to the injection mixers are provided with valves just like the injection liquid feed lines, too. Exactly the same problems are involved in dividing the mixing liquid to its respective feed pipes as those discussed above in connection with the injection liquid.